Can JP barrels be used to build on any upper with good results or are they only accurate if they are press fit by JP into their upper?

Also, will the JP thermal dissipation only fit under JP handguard?

I personally consider their "thermal dissipation" little more than snake oil, and a barrel is a barrel.  Whether it will be more or less accurate is up to you and what you put it in.

People get wonky about a chrono hanging off their barrel messing up harmonics.....and then JP bolts on a radiator

Yes, they can be used on a standard upper. The juju about "thermal press fit" has been out for awile now. Use a BCM upper with practically any barrel and you will have to/should warm it up a bit to get the barrel extension to slide into the reciever. Presto, "Thermal fit".

I've had one JP 22" 6.5 Grendel barrel which which was a consistant 1.25" performer and quite overgassed. This rifle used a thermal dissipator under a Aero Precision Handguard. Whatever one comes with the monthly builders kits. Looked cool, can't speak for how effective it was if at all.

One 16" 223 Wylde complete JP rifle that I took on a trade in that was a consistant 1.5" rifle with 77gr match ammo.

I am local to JP and shoot with JP employees and a lot of competitors that use JP barrels and rifles. My main competition rifle has a JP 14.5 barrel.

I think they are nice barrels, but not sure they are worth the high price point. The high price point isn't totally crazy though since it comes with one of their bolts. My JP barrel with 77 grain ammo is consistently about .9 to 1 MOA. My Criterion Core barrels seem to be more accurate for slightly less.

I am not sure what they do to their guns they build, but I have seen some of their factory rifles shoot pretty good groups. My friend also has a 14.5 JP, but it is a full JP gun. His easily shoots .5 MOA.

I will say that they are a very pro 2A company that supports shooting matches all over the country. They are also very supportive of MIL and LE.